The traditional way to remove unwanted harmonics from a continuous wave (CW) signal, and thereby linearize the CW signal, is to filter the CW signal after amplification. Exemplary apparatus 900 for doing this is shown in FIG. 9 and comprises a signal synthesizer 902, a non-ideal amplifier 906 and a switched filter bank 918. In operation, the CW signal synthesizer 902 receives an indication of a desired CW signal frequency (at input freq) and a reference signal (at input ref), and in response synthesizes a CW signal 904. The CW signal 904 is then amplified by the amplifier 906, and the amplified CW signal is filtered by a filter of the switched filter bank 918.
When the frequency of the CW signal 904 changes (i.e., because of a change in the desired CW signal frequency), the frequencies of the harmonics produced by the amplifier 906 also change. That is, the 2nd harmonic is located at 2*fc, the third at 3*fc, and so on (where fc is the frequency of the CW signal 904). Because the filters 910, 912, 914, 916 of the switched filter bank 918 are typically built with passive components, and have fixed passbands and stopbands, changes in the harmonics produced by the amplifier 906 require different filters 910, 912, 914, 916 to be switched into the signal path between the amplifier 906 and the filtered CW signal output 922 (e.g., via switches 908 and 920).
The above approach works well so long as the unwanted harmonics produced by the amplifier 906 are sufficiently suppressed by the filters 910, 912, 914, 916 of the switched filter bank 918. However, if the filters 910, 912, 914, 916 do not provide adequate filtering over a wide enough range of frequencies, the performance of the apparatus 900 shown in FIG. 9 can suffer.